How do I calculate True Airspeed? Can’t I just lookup the info in my POH?
Have you ever wondered just were those numbers come from that are posted in the old and trusted POH performance section? They come from years of experience and testing of various designs from the manufacture and a little physics thrown in for good measure.
At the time that Beechcraft decided to manufacture our line of aircraft, they had multitudes of data from years of previous aircraft designs to fall back on. After the aircraft was designed and the prototype was manufactured, someone had to test it. It was during this phase of the manufacturing process that our POH came to life.
During the next few months, I will post some news articles describing the methods and procedures used to obtain the data used in our POH. The fun part of all of this will be that you can use these procedures to test and verify your own aircrafts performance numbers today, using the same procedures and methods that Beech used.
My intent is not to make you all test pilots or flight test engineers, but to give you an understanding of just how these numbers came about and why, and to give you a method to verify those numbers today. The techniques I will describe will require for the most part, two people. One to collect data, and the other to fly the plane.
I thought that the first article should be something that most of us could use right away.
True Airspeed Calibration.
I see articles on the MM web site as well as other publications about Airspeed Performance for our aircraft and how slow our aircraft seem to be. The following techniques should help you to verify your True Airspeed and Range performance data and compare it to the POH.
There are two flight test techniques that can be used for airspeed calibration. They are the Ground Course and GPS (Global Positioning System) PEC (Pitot-static Error Corrections) methods. These techniques allow comparison of data from the airspeed indicator to actual ground speed.
Let’s start with the Ground Course Method.
The ground course technique, should be a surveyed line of at least three miles. The use of roads or highways are best for this technique. By noting the time to fly a known distance, actual ground speed is determined.
The Federal Aviation Administration requires that aircraft airspeed indicating systems and static pressure systems meet specific accuracy requirements. These requirements are stipulated in the Federal Aviation Regulations, Part 23, Paragraphs 1323 and 1325, Part 25, Paragraphs 1323 and 1325.
The ground course is flown at relatively high airspeed in the clean configuration. Prior to engine start, you should note the pressure altitude (altimeter set to 29.92 in. Hg.), outside air temperature, fuel on-board and gross weight.
Ground Course:
The ground course technique is normally used for low speed aircraft. Time is recorded over a known distance to get ground speed. The course is flown at least twice for each speed and altitude in opposite directions to obtain true airspeed, by negating the effect of wind. It is assumed that the same wind velocity and direction exists for both runs.
If a crosswind exists, the aircraft must be allowed to drift downwind. By doing this, it will ensure that the component of ground speed parallel to the course includes the total airspeed the aircraft would have in still air.
For each altitude flown, note the temperature and pressure altitude. At the completion of the test runs, you can use this information to determine the density altitude flown.
By averaging the “ground speed” component along the course for the two runs, true airspeed is obtained. The test will normally be accomplished in at most, light wind conditions to minimize errors. It is critical that the indicated airspeed remain constant while determining the elapsed time between markers and that exactly the same speed be used for both passes.
Altitude must be maintained. Variations in altitude will affect the outcome of the test.
The following procedures should be used while flying the ground course:
Locate your ground course way points to be used. Use cross-roads or other landmarks and have at least a three mile distance to be flown.
Perform an approximate “trim shot” at the highest indicated airspeed before you enter the course.
Enter the course at the first target altitude and waypoint and start timing. DO NOT CORRECT FOR WIND DRIFT. Allow the aircraft to drift. Do not correct for this drift. Fly using the DG or compass.
At the completion of the first pass, note the total time to travel the three miles.
Example:
ALTITUDE 4000’ Pressure Altitude OAT 82° IND A/S 115mph
PASS 1 90sec
Perform a course reversal maneuver and enter the course in the opposite direction at trim speed. Remember to let the aircraft drift in a crosswind.
Start timing with a stopwatch at the first marker, maintaining airspeed, and note the time.
Example
ALTITUDE 4000’ Pressure Altitude OAT 82° IND A/S 115mph
PASS 1 90sec
PASS 2 92sec
Perform a course reversal maneuver and repeat. Your data should look something like this:
ALTITUDE 4000’ Pressure Altitude OAT 82° IND A/S 115mph
PASS 1 90sec
PASS 2 92sec
PASS 3 89sec
PASS 4 88sec
Total 359sec
÷ 4 Number of Passes
89.75sec
You have flown a three mile course so divide 89.75sec by three = 29.92sec / Mile
There are 3600 sec in an hour so divide 3600 by 29.92 = 120.3mph. TAS at 4000’ PA
Perform this for each altitude. I use 2000’ increments (2000, 4000, 6000, 8000 etc)
The next procedure is a little easier to do.
GPS PEC method
In the GPS PEC method, GPS ground speed is used to obtain actual true airspeed. This technique is another way to do a ground course. To do this method, at least three legs are flown at the same airspeed and altitude. Using a handheld GPS, note ground speed and ground track on each leg.
Assuming true airspeed, wind speed and wind direction are constant on the three legs, then three equations in three unknowns can be solved, giving wind speed, direction, and true airspeed. The calculated true airspeed is then compared to the indicated speed and the position error is determined. To fly this maneuver:
Perform a stable trim shot at the desired airspeed and configuration. Pick an altitude and geographic location where you would expect steady wind.
Once stable, note everything that would affect true airspeed (indicated airspeed, pressure altitude, and outside air temperature).
After allowing sufficient settling time (perhaps 10 seconds after the aircraft is stable, no change in ground speed or track), record the GPS ground speed and track.
Turn 60° to 120° and repeat each step at the same airspeed and altitude.
Small changes in altitude are much preferred over any change in airspeed. A one knot speed error will produce one kt or more error in true airspeed, but a 100 ft error in altitude will have a relatively insignificant error on true airspeed. A constant ground track is important so use the DG for maintaining track.
Turn again and repeat at the same airspeed and altitude. Three legs are required to complete one data point (one speed/configuration). If four legs are done at the same conditions then the data can be averaged taking four legs three at a time, four different ways.
Remember to perform a Density Altitude calculations after you complete your tests. By using the OAT and Pressure Altitude, you can determine the Density Altitude for the tests. This is important. The Density Altitude is the altitude the aircraft is performing at.
At 4000’ pressure altitude at 82°F the aircraft will perform as if it were at 6349’ on a standard day and temp. So you would look at your POH for an altitude of either 6000 or 6500 feet and compare your TAS numbers.
Here are some useful links:
http://www.lowapproach.com/e6b.html
http://www.dfrc.nasa.gov/Education/OnlineEd/Intro2Flight/
I hope this will help in giving you some insight on True Airspeed and how to calculate it for your aircraft. Next time we will dive into Cruise Performance and Endurance.
Until Next Time.
Jeff